Process for recovering normally solid thermoplastic polymers from solution



Feb. 3, 19.70 r....-|||NTor-1 3,493,553

PROCESS FOR RECOVERING NORMALLY SOLID THERMOPLASTIC-- POLYMERS FROMSOLUTION Filed Jan. 2s, 196e;l

United States Patent O 3,493,553 PROCESS FOR RECOVERING NORMALLY SOLIDTHERMOPLASTIC POLYMERS FROM SOLUTION Robert A. Hinton, Bartlesville,Okla., assignor to Phillips Petroleum Company, a corporation of DelawareFiled Jan. 28, 1966, Ser. No. 523,732 Int. Cl. C08f 1 88 U.S. Cl.260-93.5 S Claims ABSTRACT F THE DISCLOSURE Entrained solids in solventvapor and inert -gases being removed from a polymer flash-chopping zoneare separated from said solvent vapor and inert gases prior to theremoval thereof from said zone by subjecting the mixture of solids,gases and vapor to the action of centripetal forces generated in saidzone whereby the solids therein are removed and the remainingessentially solidfree gases are then removed from the system.

This invention relates to the production and recovery of normally solidpolymers. In one aspect it relates to the recovery of a normally solidpolymer from a solution thereof in a solvent. In another aspect itrelates to a. novel method and apparatus for the production of asubstantially solvent-free polymer.

Several different processes are known in the art for the production ofnormally solid polymers, such as polyethylene, polybutadiene andpolystyrene. In many of the known processes, the polymer is initiallyobtained in the form of a solution in the solvent and must be recoveredtherefrom. Recovery can be effected by different methods, such assolvent vaporization, which leaves the polymer as a non-volatileresidue, and cooling of the solution to cause precipitation of thepolymer which is subsequently recovered by filtration. Vaporizationprocesses for solvent removal are accompanied by difficulty in removingthe last traces of solvent from the polymer. This difficulty arises fromthe fact that as the solution becomes more and more concentrated, itsviscosity increases, Heat transfer is correspondingly retarded and, ifproper control is not exercised, the polymer may be thermally decomposedas a result of unduly high residence times in recovery equipment at highlocalized temperatures. It is highly desirable to free the polymer fromthe solvent inso far as possible because small amounts, e.g. a fewwei-ght percent, of solvent in the polymer form bubbles, andconsequently cavities, when the polymer is heated preparatory tomolding.

The present invention provides a process and apparatus by which polymercan be recovered from a solution thereof by vaporization of the solventwithout thermal decomposition of the polymer and the finally recoveredpolymer contains less than 1 weight percent of solvent, often less than0.2 weight percent, and can be molded without undue formation ofcavities caused by solvent vaporization.

The process of the present invention is effected as follows. In a firststage a solution of polymer is flashed in a vaporization zone maintainedbelow the melting point of the polymer so as to remove the solvent andobtain a solid cencentrate. The flash zone has disposed therein achopping means wherein the solid concentrate is reduced to apredetermined size. In a second stage, the chopped solid concentratefrom the first stage is subjected to a mixing action countercurrent to astream of inert purge gas at a temperature below the melting point ofthe polymer and a substantial portion of the remaining solvent isvaporized. Between the first and second stage is placed the novelturbine of this invention, as will be described hereinafter, whichserves to remove rice fluid from the apparatus as well as separate theentraincd solids from the fluids. The polymer then vfreed of solvent isrecovered as product.

The melting point of the polymer will vary, depending on the physicaland chemical nature and origin of the polymer. Polyethylenes ordinarilyrange in melting point from about 210 to about 260 F. The polyethylenesprepared by the process of Hogan and Banks, U.S. Patent 2,825,721,ordinarily have melting points in the range of 240 to 260 F. but canhave melting points outside this range.

A special feature of this invention resides in conducting the flashingof the cencentrated polymer solution, in the first step, at atemperature below the melting point of the polymer. Thus, an apparentlydry or solid flash residue is obtained which is readily removed from theflashing apparatus. It is also within the scope of this invention toform and collect the sized residue in the dry state and melt thecollected residue for transfer, as a liquid, to a subsequent step in theprocess.

The recovery process of this invention is particularly applicable to therecovery of a polymer produced in a process of the type disclosed andclaimed in the aforementioned Hogan and Banks patent, wherein analiphatic l-olen having a maximum chain length of 8 carbon atoms and nobranching nearer the double bond than the 4-position Ais subjected topolymerization conditions in the presence of a catalyst comprisingchromium oxide, of which a substantialportion of the chromium ishexavalent, associated with at least one oxide selected from the groupconsisting of silica, alumina, zirconia and thoria. The chromium (Cr)content of the catalyst is ordinarily a minor proportion, preferablyfrom 0.1 to 10 weight percent. Polymerization is ordinarily conducted ata temperature in the range of to 450 F. The reacted olefin, e.g.ethylene and/or propylene, is often, though not necessarily, subjectedto the polymerization conditions in admixture with a hydrocarbon solventwhich is inert and can exist as a liquid at the polymerizationtemperature. Suitable solvents of this class are normally liquidnaphthenes, such as cyclopentane, cyclohexane, methylcyclopentane,methylcyclohexane and parafiinic hydrocarbons having from 3 to 12,preferably 5 to 12, carbon atoms per molecule, e.g. normal hexane,isoheptanes, normal octane and 2,2,4-trimethylpentane. The reaction canbe conducted as a fixed-bed reaction but is, in many cases, conductedwith the catalyst in comminuted form in suspension, as a slurry, in thehydrocarbon solvent. The efiluent withdrawn from the reactor comprises asolution 'of polymer in the solvent, and when a slurry or suspendedcatalyst is used, the eflluent comprises a solution of the polymer inthe solvent, suspended catalyst with undissolved polymer adheringthereto, and in mam1 cases, small amounts of unreacted olefin. Theunreacted olefin can be removed by venting and/or flashing and theremaining mixture is ordinarily heated to a suitable temperature toeffect substantially complete solution of the polymer in the solvent.Additional solvent can be added at this point, if desired, and it isordinarily the practice to adjust the concentration of polymer in thesolution to a value in the range of approximately 2 to approximately l0Weight percent. The solution can be then filtered to remove thesuspended catalyst and a solution containing from 2 to 10 weight percentof polymer in the solvent is obtained from which the polymer isrecovered. The method of this invention is particularly applicable tothe recovery of polymer from such a solution, and particularly to such asolution of a polyethylene. Polymers so recovered are useful forfabrication into pipe, tubing, electrical insulation and water-resistantwrappings, as more fully set forth in the cited patent.

According to one modification of this invention, approximately 25 to 75percent of the solvent is vaporized in a preliminary vaporization zone,from about 85 to 99 percent of the remaining solvent is vaporized in thefirst stage, and most or all of the remaining solvent is vaporized inthe final stage.

When the invention is applied to a solution of polyethylene in, forexample, 2,2,4-trirnethylpentane or cyclohexane, as previouslyindicated, it is preferred that the solution fed to the first stage ofthe process have a polyethylene concentration in the range 8 to 60weight percent. The first stage concentration effects a furtherconcentration to from 50 to 99 percent, and the final stage decreasesthe solvent concentration to less than 1 weight percent in the productpolymer.

In one embodiment of the invention, a preliminary evaporation isconducted at temperatures in the range 250 to 350 F. and above themelting point of the polymer and the normal boiling point of thesolvent, and a pressure in the range of to 100 p.s.i.g.; the first stepis a flash conducted at a temperature in the range 100 to 250 F. andbelow the melting point of the polyethylene, at a pressure which ispreferably not greater than 25 p.s.i.a. but which can be as high asabout 45 p.s.i.a.; the final step is a vacuum extrusion step conductedat a maximum pressure of about 10 p.s.i.a. and a temperature in therange 375 to 450 F. In the final step, the molten polyethylene issubjected to mixing or kneading and any remaining solvent is vaporizedunder vacuum. The molten polymer is then extruded in the form of acontinuous column or filament and can be cut into pellets, cylinders orother desired shape and any desired length.

In another embodiment of the invention wherein a high-solids reactor isemployed to produce the polymer, the preliminary evaporation step andsolids removal step are omitted as the polymer solution at a pressurelof 200 to 450 p.s.i.g. and a temperature of 290 to 350 F. is feddirectly into a flash zone provided with the chopper and maintained at apressure of 0 to 45 p.s.i.a. and a temperature in the range of 180 to260 F. below the melting point of the polymer.

The process of this invention can be conducted in evaporators or vacuumflash tanks, the general construction of which is well known in the art.However, certain special features are involved in the construction ofthe flash apparatus. It has been found that it is difficult to handleconcentrated polyethylene due to low bulk density of the polymer formedafter flashing. This condition can be remedied by one aspect of thisinvention which provides novel mechanical means for severing theextruded strings or filaments. The apparatus for this purpose issubseqeuntly described herein.

The apparatus used in the final step is ordinarily in the form of aheated elongated chamber having apparatus connected therewith which canreduce the pressure therein to subatmospheric. This apparatus isprovided with an agitation or kneading device, such as one or more screwconveyors, and is further equipped with one or more outlet conduits inthe form of a constricted opening or die through which the moltenpolymer can be extruded. A suitable device for the final stage solventremoval is manufactured by Welding Engineers, Inc., of Norristown,Pennsylvania, and is described in bulletins currently published by thatfirm. Such an apparatus is capable of operating at a pressure as low as6 millimeters of mercury, absolute.

The solvent vaporized in the steps according to this invention can becondensed, combined and recovered, for example, for recycling to thepolymerization reaction.

Other objects, advantages and features of the invention will be apparentto those skilled in the ait from the following description, the drawingsand the appended claims.

In the drawings:

FIGURE l is a diagrammatic illustration of the invention.

FIGURE 2 is a diagrammatic illustration of the solid polymer recoveryzone of FIGURE 1.

FIGURE 3 is a diagrammatic view of the turbine shown in FIGURE 2.

Referring now to FIGURE 1 of the drawing, solvent enters through inlet 2and is mixed with catalyst supplied from storage zone 3. The catalystcan be, for example, chromium oxide supported on a silica-alumina geland prepared as described subsequently herein. The catalyst particlesize is sufficiently small to facilitate the formation of a slurry orcatalyst in the solvent. A suitable range of particle size is from 20 to100 mesh. The catalyst-solvent slurry passes into reaction zone 4.Ethylene enters the system through inlet 5 and passes into the reactionzone 4 wherein it is mixed with the catalyst and the solvent at atemperature, for example, of approximately 275 F.

The solvent can be cyclohexane. The proportions of solvent and ethyleneare so adjusted that the concentration of polymer in the reactionmixture does not exceed approximately 15 percent and preferably is inthe range from 5 to l0 weight percent. However, when a high-solidsreactor is employed as described hereafter, the polymer concentration isgenerally about 40 weight percent. The pressure in the reaction zone 4is sufiicient to maintain the solvent substantially in the liquid phaseand can be, for example, 500 p.s.i. The reaction mixture is maintainedin a state of turbulence so that the catalyst is maintained in asubstantially uniform suspension or slurry in the reaction mixture. Thisturbulence can be obtained by jet action of incoming ethylene throughinlet 5 and/or by the use of a mechanical stirrer indicated by thenumeral 6 and driven by a suitable motor M. The reaction zone efiiuentwhich comprises a mixture of polymer, solvent and suspended catalyst,together with small amounts of unreacted and/or inert gas, is passedthrough conduit 7 and heater 8 to dissolution zone 9. Additional solventcan lbe added, if desired, in order to adjust the concentration to asuitably low value, previously stated, so that the viscosity is not toohigh for efficient agitation. In dissolution zone 9, the mixture ismaintained in a state of turbulence as, for example, by means of amechanicall stirrer 13 driven by a motor M and the temperature ismaintained, for example, by the use of heater 14, at from 300 to 325 F.,i.e., somewhat higher than that utilized in reaction zone 4. Thepressure is sufiicient to maintain the solvent substantially in theliquid phase but is preferably lower than that in reaction zone 4 to-facilitate the evolution of dissolved gas, including unreactedethylene, which is vented through outlet 15. Heater 14 is of anysuitable design known in the art; for example, it can be a steam coil oran electric immersion heater. Efiiuent from dissolution zone 9 passesthrough conduit 10 to solids removal zone 16. The material passedthrough conduit 10 is a homogeneous solution of substantially all of thepolymer in the solvent, which solution contains suspended solidcatalyst. Solids removal zone 16 comprises any suitable equipment orcombination thereof known in the art for the removal of suspendedsolids' from liquids. For example, it can be a filter or a centrifuge.It should be suitable for operation under pressure in order to maintainthe solvent in the liquid phase during the' filtration. Catalyst removedby the filtration is withdrawn from the system through conduit 17. Thewithdrawn catalyst can be regenerated or reactivated, if desired, andrecycled to catalyst storage zone 3 by means not shown in the drawing.The solution which has been freed of suspended solids is passed throughconduit 18 to preliminary evaporation zone 19 which is ordinarily in theform of a flash evaporation tank and is operated, for example, at atemperature of 290 F. and a pressure of 33 p.s.i.g. Approximately halfof the solvent is evaporated in zone 19, and the evaporated solvent ispassed through conduit 20 and condenser 21. The condensed solvent isthen returned through conduits 22 and 2. The concentrated residue fromevaporation zone 19 is passed through conduit 23, pump 24 and heatexchanger 25 wherein the temperature is raised, for example, to 310 F.Part of the solution is returned through conduit 26 to evaporation zone19. This mode of operation allows outside heating of the unvaporizedmaterial from evaporation zone 19 and is a preferred method of supplyingheat to said zone, since it is ordinarily impractical to supply heatefficiently directly to the interior of zone 19.

The remainder of the unvaporized material is passed through conduit 27to flash zone 28 which is operated, for example, at a temperature of 180to 260 F. and 0 to 45 p.s.i.a. The solution entering ash-comminutionzone 28 has a concentration, for example, of l() to 60 weight percentpolyethylene in the cyclohexane solution. The partially concentratedpolymer solution enters ash zone 28. If flash zone 28 is operated undervacuum, pump 33 is used to remove vapors from accumulator 36. If zone 28is operated under pressure, pump 33 is not used. Solvent which isvaporized in ash zone 28 is condensed in condenser 34 and passed throughconduit 35 to accumulator 36. Gaseous material is Withdrawn from thesystem through exhaust conduit 38. Condensed solvent is passed throughconduit 39 by means of pump 40 and then passes through conduits 41 and22 for return to conduit 2.

Flash zone 28 has therein a knife-carrying member 62 with three or morearms, each of which carries a knife blade. Screen 64 is provided ofappropriate size to retain the solid polymer in zone 28 until thedesired polymer particle size as determined by the size of the screen 64is obtained by means of the chopper 62.

The sized solid polymer particles having fallen through screen 64 arecollected in purge zone 65. Inert gas such as nitrogen is introducedthrough line 72 into the zone 65 so as to further assist in removingsolvent remaining in the polymer particles. The level within zone 65 iscontrolled by level controller 58 which is operatively connected tomotor 30 and star valve 31 and thereby regulates the rate of removalfrom zone 65.

Turbine-type centrifugal separator 78 is horizontally placed belowscreen 64.

Because of the higher pressureV in purge zone 65, purge gases andvaporized solvent containing some entrained solids pass into the inletof turbine separator 78. The centrifugal action of separator 78 removesthe solids from the vapor, throwing said solids back into purge zone 65.The solids-free vapor is then withdrawn from the purge zone asillustrated in FIGURE 3. The vapor is then passed via line 35 tocondenser 34 as previously described.

The material from purge zone 65- in communication with vacuumextruder-dryer 43 passes to same. This vacuum extruder-dryer comprisesan inner chamber 44 enclosed within a heating jacket 45 through Iwhichhot oil is circulated, being supplied through inlet 46 and withdrawnthrough outlet 47 to heating and recirculation means, not shown. Withinchamber 44 is positioned one or more helical conveyors or extruders 48mounted on a shaft which is connected with a suitable driving motor, asshown in the drawing. Also connected with the inner chamber 44 of thevacuum extruder-dryer through conduit 50, condenser 51, conduit 52,accumulator 53 and conduit 54 is vacuum pump 55 which exhausts throughconduit 56.

Polymer at a temperature below its melting point is Withdrawn at a rate-regulated by level controller 32 and star valve 31 from purge zone 65to vacuum extruderdryer 43 and is therein kneaded at a temperature aboveits melting point, under a vacuum produced by vacuum pump 55, so thatsubstantially the last trace of solvent is yremoved therefrom. Solventvaporized in vacuum extruderdryer 43 is passed through conduit 50,condensed in condenser 51 and passed into accumulator 53. Liqueedsolvent is then withdrawn and returned through conduit 41 and pump 57through conduit 22 to inlet 2.

Molten polymer is extruded as one or more strands or filaments fromvacuum extruder-dryer 45 along or through conduit or route 59 andsubsequently recovered in a conventional manner.

In another embodiment of this invention, as further shown in FIGURE 1,when a high-solids reactor such as that described in copendingapplication Serial No. 208,- 047, tiled July 6, 1962, is employed thepolymer concentration in the cyclohexane solvent is about 40 kweightpercent. Thus the dissolution zone 9, solids removal zone 16 andpreliminary evaporation zone are omitted from the ow path and thereactor eluent is introduced by means of conduits i60 and 61 directly tothe ash zone 28 and ashed therein. The solid polymer is thereaftertreated as previously described.

As shown in FIGURE 2, a polymer pelletizing unit is provided whereinconduit 27 is provided to introduce the polymer solution into the flashzone 28. Conduit 35 is provided to remove purge gas introduced throughline 72 and the solvent flashed from the chopping-flash zone. Thepolymer precipitated is reduced in size by the chopper 62. The wall ofthe apparatus supports approximately at its center a horizontal shaft 66which revolves in suitable bearings and supports in its inner part aknife-carrying member -which is virtually integral with it. This memberhas three or more arms 68, each of which carries a knife securedthereon. The knife-carrying member has discs on both sides to preventparticles of the material being disintegrated from penetrating to thebearings of the machine. On a prolongation of the shaft 66 is mounted aflywheel which serves at the same time as a driving pulley.

Around the knife-carrying member are arranged a certain number of fixedknives 71 mounted on the body of the apparatus. The distribution of thefixed knives is such that in no case is a cut effected between more thanone xed knife and one rotating knife at the same time, the efficiency oroutput of the machine being thereby increased and the motive powerrequired being reduced.

The comminuted material and vapor passes through a semi-cylindricalperforated screen I64 in a downward direction after being reduced bychopper '62 t0 a size capable of passing therethrough. A turbine-typecentrifugal separator 78 is placed below screen 64 for removal ofsolvent vapor and inert lpurge gas. Removal of the iluid is effected bya slight pressure drop across the turbines vanes as illustrated inFIGURE 3. The swirl imparted to the fluid while passing through theturbine vanes effectively centrifuges any particles and returns sameinto the accumulation zone 65. The turbine lwill thus permit withdrawalof a fluid stream essentially free of entrained solids. A specialadvantage of the turbine separator over other gas-solid separators suchas cyclones is the selfcleaning action of the turbine. There is notendency for the turbine to foul by accumulation of polymer on theturbine parts. In contrast, a cyclone will quickly foul by deposition ofpolymer on the cyclone walls.

Another advantage of the turbine separator over other separators is thatthe separation eficiency of the former can be adjusted to suit therequirement, i.e., the turbine speed, vane dimensions, eflluent gasvelocity, etc., can be varied to increase or decrease the eiciency ofseparating solids from gases. The particle sized polymer is collected inpurge zone 65 which is in communication at the lower end thereof with anevaporator-dryer such as shown in FIGURE 1. `Conduit 72 is provided tointroduce purge gas into the lower portion of the purge zone 65.

The level in zone 65 is regulated by 'means of a level detector such asthe radiation type level detector 31 which controls the on-off action ofstar valve 32 by passing a signal to motor 30, which in turn actuatesthe valve member. The illustrated level controller has a radiationsource 29 and a detector means such as an Ohmart detector 58 which, dueto changes in radiation transmission across the column due to thepolymer level within zone 65, provides a signal to actuate the powersource 30 for star valve 32.

In order to further assist the purging of the particulate solids in zone65, agitator 69 with drive assembly 76 is provided. The agitating meanscan be of any design and, as illustrated, comprises a shaft 69 supportedby mount 70 having a plurality of arms 74 thereon.

As shown in FIGURE 3, the turbine centrifugal separator 78 is actuatedby drive shaft 80 powered by any suitable drive means, such as motor 79shown in FIGURE 2. The turbine is placed in a horizontal positionthrough wall 82. Fluid effluent tube 84 forms a passageway for driveshaft 80 through the wall 82 and further serves to remove solids-freesolvent vapor and purge gas from the flash vessel by means of thecentrifugal force generated by the swirl of the turbines vanes 86'. Theeffluent is withdrawn axially through the turbine while centrifuging thepolymer particles back into the flash vessel. This is accomplished bymeans of vanes 86 being spaced radially around the drive shaft 80. Asthe turbine rotates the fluid is caused to swirl and the centripetalforce developed is utilized to remove the swirling solids. To assurerelatively high radial velocity imparted to all effluent ud the hub ofthe turbine is maintained at a relatively large outside diameter. Theturbine rate, rpm., and vane size can be varied to conform to theminimum rotational velocity required to centrifuge the smallestparticles encountered in any given separation.

The following examples are presented to more fully describe theinvention, but it is not intended that they should be construed aslimiting the invention thereto.

EXAMPLE I In a run for the production of polyethylene, a saturatedsolution of ethylene in cyclohexane is maintained in a pressure reactorequipped with a stirrer. The cyclohexane containing 20 to 100 meshcatalyst in suspension is supplied continuously to the reactor. Ethylenefrom which oxygen has been removed by contact with reduced copper oxideis supplied to the reactor as a separate stream. The catalyst isprepared by impregnating a steam-aged, coprecipitated gel compositecomprising 90 Weight percent silica and 10 weight percent alumina withan aqueous solution of chrominum trioxide, drying the resulting solidcomposite, and heating the dried composite at approximately 950 F. forabout 5 hours in a stream of substantially anhydrous air. The catalystcontains a total of 2 weight percent chromium, at least half of which isin the hexavalent state.

The reactor is maintained at a temperature of approximately 300 F. and apressure of approximately 600 p.s.i.g. Total ellluent is continuouslyWithdrawn from the reactor, heated to 315 F. and passed to a dissolutiontank maintained at 315 F. and 100 p.s.i. from which unreacted ethyleneand any other normally gaseous material is vented. Additional solvent isadded to the total reactor effluent prior to the heating and flashing.The proportion of ethylene to total cyclohexane added upstream anddownstream from the reactor is so adjusted that a solution containingapproximately 5 weight percent of polyethylene in cyclohexane isobtained. After heating the effluent to approximately 315 F. andagitating to effect complete solution of the polymer in the solvent, asdescribed, the catalyst is removed by filtration at approximately 315 F.and 100 p.s.i. The resulting 5 percent solution of polyethylene ispassed to a solvent evaporator maintained at 290 F. and 33 p.s.i.g.wherein approximately half of the solvent is vaporized. The unvaporizedmaterial is withdrawn from the body of the evaporator and passed througha heater wherein it is heated to approximately 310 F. Approximately halfof the heated material is returned to the evaporator to supply heatthereto. The remainder is passed to a vacuum flash tank maintained at200 F. and 3 p.s.i.a. The entering solution, which contains about weightpercent polyethylene, is passed to a flash chopping zone whichcomminutes the material emerging from the flashed solution. The solid iSthen passed through a sizing screen to a purge zone. Vaporized solventpassing through the sizing screen and the purge gas have entrainedpolymer paricles therein. This fluid is passed through a turbineseparator which removes and directs the entrained solids back to thepurge zone. After purging with nitrogen the solid particles are passedto a vacuum extruder-dryer Model 2052B (Model 205213 Extra Long is alsosatisfactory), manufactured by Welding Engineers, Inc., and containing adouble helical agitator. 1n the vacuum extruder-dryer the temperature ismaintained between 370 and 450 F. by circulating hot oil through theheating jacket. The pressure wihin the middle chamber of theeXtruder-dryer is maintained at approximately 30 to 50 mm. Hg. Moltenpolymer containing from 0.019 to 0.026 weight percent of solvent isextruded from the vacuum extruder-dryer and passed through an open tankcontaining water which cools and solidities the polymer. The solidifiedpolymer emerges from the cooling tank and is cut by means of a rotarycutter into cylindrical pellets which are recoverd as the product of theprocess.

EXAMPLE II Ethylene is fed into the reactor at the rate of about 76,749pounds per day at a temperature of about 230 F. Activated catalystconsisting essentially of chromium oxide (a portion hexavalent)deposited on silica alumina (chromium oxide concentration about 2 weightpercent) in finely divided form, 50 weight percent being less than 10microns in size, is admixed with cyclohexane (as the solvent feed) andthe resulting catalyst slurry is lfed to the reactor at the rate of 14.4pounds of catalyst and 160,000 pounds of cyclohexane per day. By meansof circulating the cooling water through the jacket of the reactor, thetemperature is maintained at about 260 F. and the reactor pressure iscontrolled at about 450 p.s.i.a.

The eflluent solution of polymer, containing about 32 weight percentpolymer, is fed directly to the chopper zone Which also serves as flashzone. The entering solution contains about 32 Weight percentpolyethylene. The material which emerges from the flashed solution ischopped and passes through a sizing screen to a purge zone. Vaporizedsolvent passing through the sizing screen and the purge gas haveentrained polymer particles therein. This fluid is passed through aturbine separator which separates and directs the entrained solids backto the purge zone. After purging with nitrogen the solid particles arepassed to a vacuum extruder-dryer Model 2052B (Model 2052B Extra Long isalso satisfactory), manufactured by Welding Engineers, lnc., andcontaining a double helical agitator. In the vacuum extruder-dryer thetemperature is maintained between 370 and 450 F. by circulating hot oilthrough the heating jacket. The pressure within the middle chamber ofthe extruder-dryer is maintained at approximately 30 to 50 mm. Hg.Molten polymer containing from 0.019 to 0.026 weight percent of solventis extruded from the vacuum extruder-dryer and passed through an opentank containing water which cools and solidies the polymer. Thesolidified polymer emerges from the cooling tank and is cut by means ofa rotary cutter into cylindrical pellets which are recovered as theproduct of the process.

Although the process of this invention has been described in connectionwith particular polyethylene processes, it is clearly not limitedthereto but is also applicable to the recovery of any normally solidthermoplastic polymer from a solution thereof in a solvent. Thus, thepro-cess is also applicable to solutions of polybutadienes, especiallyhydrogenated polybutadienes as described in U.S. Patent No. 2,864,809 byJones and Moberly, polystyrenes, polypropylenes, polyisobutylenes, andpolyethylenes produced by processes other than that of the typedescribed herein, as well as to the recovery of halogenatedpolyethylenes. Also, the process is not limited to the recovery ofpolymers from saturated hydrocarbon solvents but is applicable whereinsolvents such as chloroform, carbon tetrachloride, carbon disulde andaromatic hydrocarbons and derivatives thereof are used as solvents. Theessence of this invention is that a normally solid thermoplastic polymercan be recovered substantially free from solvent by a process whichcomprises evaporating a substantial portion of the solvent at atemperature below the melting point of the polymer while simultaneouslychopping or comminuting same, agitating the polymer at a temperatureabove its melting point to vaporize the remaining solvent, andrecovering a substantially solvent-free polymer.

Reasonable variations and modifications of this invention can be made orfollowed, in View of the foregoing, without departing from the spirit orscope thereof.

I claim:

1. In a process for recovering a normally solid thermoplastic polymerfrom a solution thereof by means of the steps comprising ashing saidsolution at a temperature below the melting point of said polymer in achopping-ash zone so as to remove solvent therefrom as a vapor andthereby produce a solid polymer, sizing the resulting solid polymer bychopping said solid polymer in said chopping-flash zone, purging theresulting sized particles of polymer by contacting same with an inertgas in a purge zone maintained in open communication with said flashzone, removing a stream of vapor consisting of a mixture of said solventvapor and purge gas from said chopping-flash zone, mixing the resultingsized polymer particles at a temperature above its melting point so asto remove essentially all of the remaining solvent therefrom,solidifying solvent-free polymer and thereafter recovering the resultingsolid, substantially solvent-free polymer as a product of the process,the improvement comprising applying centripetal forces to said stream ofvapor consisting of a mixture of solvent vapor and purge gas while beingremoved from said chopping-Hash Zone whereby entrained solids areseparated from said vapor and retained in said zone so as to effect theremoval of an essentially solid-free vapor from said chopping-Hash zone.

2. A process according to claim 1 wherein said polymer is polybutadiene.

3. A process according to claim 1 wherein said poly mer is polyethylene.

4. A process according to claim 1 wherein said polymer is polystyrene.

5. A process according to claim 1 wherein said polymer is a normallysolid polymer obtained by polymerizing at least one l-olefin having 2 to8 carbon atoms.

References Cited UNITED STATES PATENTS 3,072,626 l/l`963 Cines.

3,013,005 12/1961 Solvik.

3,024,228 3/ 1962 McLeod.

3,251,428 5/1966 Tabler 159-48 3,290,278 12/ 1966 Rice et al. 159-48JAMES A. SEIDLECK, Primary Examiner U.S. Cl. X.R.

